1. Field of the Invention
This invention relates to an ink jet head manufacturing device, an ink jet head manufacturing method, and an ink jet device, which are appropriate for performing depolarization or polarization processing of a piezoelectric element as an ink-pressure control part in an ink jet head.
2. Description of the Related Art
A piezoelectric element is known as an ink-pressure control part for use in an ink jet head, and supplying a voltage to the piezoelectric element causes a distortion to be produced in the ink jet head. After baking of a piezoelectric element is performed, polarization processing of the piezoelectric element is performed by supplying a high voltage, which is higher than a normal driving voltage, to the electrodes of the piezoelectric element. By this process, the piezoelectric element shows good piezoelectric characteristics to a specific direction, and it can be used as an actuator in the ink jet head.
Conventionally, after polarization of the piezoelectric element is done, an ink jet head is assembled with the piezoelectric element. However, in order to connect a film substrate on which a control IC is mounted, with the piezoelectric element, soldering, an anisotropy electric conduction film, etc. is used, and heating of the piezoelectric element must be performed at the time of connection of the film substrate and the piezoelectric element. If a temperature of the piezoelectric element during the heating process reaches the Curie temperature, the piezoelectric characteristics of the piezoelectric element will be deteriorated.
To avoid the problem, there is a proposed method of connecting the film substrate on which the control IC is mounted to the piezoelectric element (see Japanese Laid-Open Patent Application No. 2003-55045). In the proposed method, a piezoelectric element having a comparatively high Curie temperature is selected and the connection of the film substrate and the piezoelectric element is performed at a temperature lower than the Curie temperature of the selected piezoelectric element.
However, in recent years, there is the demand for using a solder containing no lead, and, if such a solder is used, a temperature needed for the connection of the film substrate and the piezoelectric element is raised to a higher temperature. Moreover, there is also the demand for using a high temperature at which the heat treatment is conducted, in order to improve the bonding strength and ensure the surface treatment of the nozzle surface or the liquid passage. Selection of a piezoelectric element having a high Curie temperature which is above a desired temperature will result in a problem that the range of selection of a piezoelectric element must be narrowed (see “Nikkei Electronics”, No. 635, page 95, issued on May 8, 1995 from Nikkei BP Publication, and “Material Characteristics of Piezoelectric Ceramics”, Material-Characteristics Table, in December, 2007 from URL: http://www.fujicera.co.jp/product/j/01/mat.table_j.pdf>).
When a piezoelectric element is cut into a comb-tooth pattern, or when an ink jet head is assembled, there is a problem that the nozzles of the head have variations in the driving force, due to the factors, including the electric factor, the finishing accuracy, and the assembly accuracy. To avoid the problem, there is a proposed method in which performing depolarization processing of a piezoelectric element is initially performed and re-polarization processing is performed with a suitable intensity individually selected for each of the respective nozzles of the head (which method will be called polarization compensation). See Japanese Laid-Open Patent Application No. 2001-277525 and Japanese Laid-Open Patent Application No. 2006-315326.
A method of performing depolarization of a piezoelectric element is to place the piezoelectric element to a temperature above the Curie temperature (see Japanese Laid-Open Patent Application No. 2001-277525 and “Piezoelectric Ceramic Technical Handbook”, page 52, in December, 2007 from URL: http://www.fujicera.co.jp/product/j/01/10.pdf).
In the case of this method, after the ink jet head is assembled, the piezoelectric element must be exposed to a high temperature of several hundreds of degrees C., which will cause the deterioration of the bonding strength of the adhesives and the degradation of the components, etc.
Another method of performing depolarization of a piezoelectric element is to supply an alternating voltage to the electrodes of the piezoelectric element. The resistance of the control unit to the reversed polarity is generally low, and if the above method is used, a problem of destroying the control unit may arise.
A commonly used method of performing polarization of a piezoelectric element is to supply a voltage, which is higher than a certain voltage, to the electrodes of the piezoelectric element (see Japanese Laid-Open Patent Application No. 2001-277525).
In the case of this method, the supplied voltage is higher than the voltage needed for normal driving, and in order to perform polarization safely, it is necessary to select a control unit having a voltage resistance against a high voltage which is higher than the normal driving voltage. Therefore, the range of selection of the control unit is narrowed and the cost is raised.
To avoid the problem, there is a proposed method (see Japanese Patent No. 3879721). FIG. 10 is a diagram showing the composition of a control circuit which performs the proposed method. In the control circuit shown in FIG. 10, a control unit is arranged to determine whether a driving signal is supplied to an ink-pressure control part, in accordance with input data. A polarization voltage is supplied to both a wiring which is electrically connected to a ground terminal of the control unit and a wiring which is electrically connected to a common electrode of the ink-pressure control part. A piezoelectric element is charged with a current which passes through a diode. The diode is arranged so that a current from the wiring electrically connected to the ground terminal of the control unit can flow into the individual electrode of the ink-pressure control part. In the proposed method, the polarization voltage is not supplied to the control unit, which prevents the destruction of the control unit. The polarization processing of all the piezoelectric elements having the common electrode is performed by the same polarization voltage.
As disclosed in Japanese Patent No. 3879721, there is no problem in supplying a direct current voltage to perform a certain polarization. However, when controlling the degree of polarization precisely, it is necessary to supply a pulse-like voltage waveform. When the voltage change of the polarization voltage is very frequent (for example, when a polarization voltage waveform shown in FIG. 11A is supplied to the common electrode of the piezoelectric element), a voltage waveform shown in FIG. 11B appears in the individual electrode of the piezoelectric element.
This is because the control circuit according to the related art shown in FIG. 10 has no path to which the electric discharge flows, and the drop of the voltage of the individual electrode depends on the natural electric discharge of the piezoelectric element.
When the voltage change of the polarization voltage is very frequent, the peak voltage Vp shown in FIG. 11B is increased and approaches the peak voltage Vpp of the polarization voltage. However, the peak voltage Vp is not larger than the value of the sum of the forward drop voltage of the diode D1 and the voltage Vcc, and the risk of destroying the control unit is not so great.
However, the probability that the control unit be destroyed will be increased, and polarization is performed by the electric field produced according to the potential difference between the common electrode and the individual electrode of the piezoelectric element. The composition according to the related art, shown in FIG. 10, has the following problem.
Depending on the value of the peak voltage Vp in FIG. 11B, the polarization may progress to some extent even after the supplying of the polarization waveform is finished. The polarization must be performed by taking into consideration the natural electric discharge of the piezoelectric element, and it is difficult to perform precise polarization. Moreover, a certain period of time is needed for the natural electric discharge, and the productivity becomes poor.
When it is intended to perform depolarization of a piezoelectric element, it is necessary to supply the alternating voltage to the electrodes of the piezoelectric element, so as to change the direction of the electric field generated between the common electrode and the individual electrode of the piezoelectric element successively.
In the control circuit according to the related art, shown in FIG. 10, there is no path to which the electric discharge flows. When the voltage change of the depolarization voltage is very frequent (for example, when a depolarization voltage waveform shown in FIG. 12A is supplied to the common electrode of the piezoelectric element), a voltage waveform shown in FIG. 12B appears in the individual electrode of the piezoelectric element. The direction of the electric field generated between the common electrode and the individual electrode does not change, and it is impossible to perform depolarization.
In recent years, an ink jet head has come to be used for industrial fields, and it is demanded to reduce as small as possible the variations in the discharge liquid drop weight for every nozzle of the ink jet head, such as in a manufacturing device of a organic EL (electroluminescent) display.
Moreover, the improvement of the printing speed of printers is being rapidly enhanced, and it is demanded to reduce as small as possible the variations in the discharge liquid drop speed for every nozzle of the ink jet head. In order to suppress the variations in the discharge liquid drop, the method of performing the polarization compensation described above is effective. However, even if the products after the polarization compensation are delivered, the problem of the variations in the discharge liquid drop arises due to the changes over time, the environmental changes, etc.